The present invention relates to a method of stacking a sheet of wood veneer and also to an apparatus for performing the method.
According to a conventional veneer sheet stacking method which is performed during a process of manufacturing plywood panels using such veneer sheets, a veneer sheet (or a plurality of veneer sheets laid one on another) is carried by a conveyer having a suitable sheet support to a stacking position, where the support is operated to open thereby to release and allow the veneer sheet (or sheets) to drop onto a pile of similar veneer sheets. Since veneer for such stacking operation is provided in the form of a sheet having a large surface area for its mass or weight, the veneer sheet is easily subjected to the influence of air resistance when it is being dropped, so that it may not fall just straight onto the pile, but placed out of position on the pile, with the result that a disorderly pile of veneer sheets will be formed. During transferring veneer sheets of such a pile to a subsequent station for any further processing, the pile and hence veneer sheets therein are susceptible to damage.
An apparatus which addressed such problem is proposed by Examined Japanese Utility Model Application Publication No. S52-22062, which is shown in FIG. 8. Reference numeral 73 on the drawing designates an endless conveyer which is operable to transfer a veneer sheet 71 in arrow direction toward the stacking apparatus. The stacking apparatus, which is generally designated by 75, is located adjacent to the downstream end of the conveyer 73 as seen in the conveying direction of veneer sheet 71. The stacking apparatus 75 has a plurality of perforated endless belts 77 (only one being shown in FIG. 8) which are operable to hold a veneer sheet by suction or vacuum and movable in arrow direction to carry such veneer sheet to the stacking position that is located immediately above a pile of veneer sheets 89 placed on a support block 81. Reference numeral 79 depicts a suction device 79 disposed in contact with the upper surface of the bottom horizontal legs of the perforated endless belts 77 for creating vacuum for holding the veneer sheet 71.
The stacking apparatus 75 further has a separating bar 83 which is usually made of steel and movable vertically reciprocally as indicated by double-headed arrow by a linkage 85 which is moved by a rotatable eccentric wheel 87 driven by a motor (not shown). The bar 83 separates veneer sheet 71 from the perforated endless belts 77 thereby to allow the veneer sheet 71 to fall toward the pile 89. As is apparent from the drawing, the separating bar 87 makes one stroke of reciprocating motion by one complete turn of the eccentric wheel 87.
Between any two adjacent belts 77 is provided a restraining band 91 having one end thereof fixed at a position that is higher than the bottom horizontal legs of the perforated endless belts 77. As shown in FIG. 8, the restraining band 91 droops by its own weight such that a majority part of the band 91 rides on the top veneer sheet in the pile 89 with the distal free end of the band 91 located adjacent to the downstream end of the bottom horizontal legs of the endless belts 77. Reference numeral 93 designates a sensor for detecting the arrival of a veneer sheet which has been conveyed by the belts 77 to the aforementioned stacking position immediately above the pile of veneer sheets 89. Though not shown in the drawing, the support block 81 has a suitable means such as hydraulic cylinder for elevating the stacking block 81 in response to a signal generated by a sensor such photocell operable to detect the top position of the pile 89 so that the top of the pile 89 always maintains a predetermined height.
In operation of the apparatus, a veneer sheet 71 moved forward by the conveyer 73 is picked up and held by the endless perforated belts 77 under the influence of vacuum developed by the suction device 79 and further conveyed. In the meantime, the veneer sheet 71 is brought into contact with the restraining band 91 and moved further under and in contact with the restraining band 91 until the leading end of the moving veneer sheet 71 is detected by the sensor 93, which then generates a detection signal. In response to the signal, the motor is activated thereby to operate the separating bar 83 so as to make one complete stoke of up and down movement as indicated by double-headed arrow. Thus, the veneer sheet 71 is separated and dropped off from the belts 77 together with the band 91 then riding on the veneer sheet 71. Because of the frictional force between the restraining band 91 and the veneer sheet 71 due to the weight of the former riding on the latter, the veneer sheet 71 is dropped substantially vertically straight without swerving or swaying and. Therefore, the veneer sheet 71 is stacked in an orderly manner onto the pile 89.
In the above-described prior art apparatus, the force acting on the veneer sheet 71 in downward direction is provided by the weight of the restraining band 91 (to be more precise, the downward force is actually smaller than the weight of the band 91 because the veneer sheet 71 is then being dropped). If the weight of the restraining band 91 is small, however, the frictional force between veneer sheet 71 and the restraining band 91 is reduced and the veneer sheet 71 cannot be prevented from making irregular movement while it is being dropped. If the weight of the restraining band 91 is increased with an attempt to restrain the movement of the veneer sheet 71, a force of greater magnitude is applied to the veneer sheet 71 when it is brought into contact with and moved under the restraining band 91 having an increased weight. As a result, the veneer sheet 71 may change its posture or be subjected to a damaging force while it is being moved by the belts 77.
Apart from the prior art apparatus of the aforementioned publication, another apparatus is known in the art in which a veneer sheet carried forward by a conveyer belt such as the belt 77 is separated therefrom by rapidly moving a bar which is similar to the bar 83 but operable to move to force a veneer sheet downward until it is pressed against the top veneer sheet in the pile. The bar is usually made of a rigid material such as steel. When the number of veneer sheets in the pile is still small, the bar may receive a damaging shock of impact when the veneer sheet is pressed by the bar against the top of the pile placed on a stacking block which is also made of a rigid material.